1. Field of the Invention
The invention relates to a priority flow control valve for hydraulic power circuits.
2. The Prior Art
Existing prior art embraces flow control valves consisting substantially in a housing with an inlet, connected to the high pressure outlet of a hydraulic pump, and a main cylindrical bore; the bore slidably accommodates a piston by which it is split into two chambers, and receives oil direct from the inlet. The first cylindrical chamber is connected with the second cylindrical chamber by way of a passage that incorporates a flow control element, so that the second chamber is in receipt of oil under pressure at a steady rate of flow for operation of a given service. Accordingly, the second chamber connects with a fixed flow outlet, supplying the priority circuit, whilst the first connects with an outlet that may either unload the oil to the tank or direct it toward another service. The piston is of length such as to block either one or the other outlet at any given moment, and associates by way of a rod, occupying the second chamber, with a second piston that is biased in one direction by a spring housed in a further chamber connected hydraulically with the second chamber.
The valve is solenoid-operated, and will be wired in such a way that energization of the coil activates the constant flow outlet, that is, the priority circuit.
Flow control valves of the type outlined above are widely utilized to operate the wrecking actuators with which excavators are equipped. It has emerged, however, that this conventional valve design betrays a number of drawbacks, one of which being that the priority outlet is not always faultlessly blocked when not in use; thus it can happen that there is a continual leakage of oil, sufficient to occasion movement of the actuator, the consequences of which can be extremely hazardous when such movement is not desirable. Indeed, manufacturers stipulate a minimum pressure rise for such valves.
In another version of this same type of flow control valve, the leakage of oil is prevented by installing a preloaded backpressure valve to balance the force of the spring. Leakage is prevented by the adoption of this expedient, certainly, though one has a further drawback in that there is a permanent pressure drop during operation of any one of the excavator's different services. The resultant power loss will automatically dictate additional heat, which must be dissipated; practical experience shows that power losses of between 2 and 7 horsepower can occur, depending on pump flow, and it is absolutely essential that this surplus heat be dispersed.
The object of the invention is to overcome the problems aforementioned, and in particular to embody a priority flow control valve in which the priority outlet is guaranteed to close efficiently, so that power losses will not occur during normal operation of the machine in which it is installed.